Balanced detector system for radar apparatus



June 6, 19 50 .1. R. MOORE swcan nsmc'roa SYSTEM FOR RADAR APPARATUS Filed Feb. 7, 1947 3110111101 JAMES R. MOORE (Ittorneg him-ten e I 2,510,710

UNITED STATES PATENT) orrica BALANCED DETECTOR SYSTEM'FOR RADAR APP ATUS James R. Moore, Washington, It. 0., assignor to Radio Corporation of America, a corporation of Delaware Application February}, 1947, Serial No. 727,174

6 Claims. ((1343-14) 'Myinvention relates to the use of reflected a transmitter antenna and as a'receiver antenna.

radio waves for obtaining information concern- The connection from the transmitter III to the ing the wave reflecting object. The invention antenna It is through a length of coaxial cable will be described with particular reference to a It, through a coaxial line section it that is a frequency modulated radar system. 5 quarter wave length long, andthrough a coaxial One of the objects of the invention is to prosection I]. vide animproved method of and means for op- The signal reflected from the distance object erating radar apparatus of the continuously is picked up by the antenna l3 and passes transmitting type with a single antenna used for through the cable II and through the quarterboth transmitting and receiving. wave line it, the received signal being applied A further object of the invention is to provide to a diode mixer l8 from the antenna end of an improved method of and means for beating the section Hi and to a diode mixer it from the a reflected signal with signal taken directly from other end of the section it.

the transmitter that transmitted said signal to The quarter-wave section It is impedance the reflecting object. matched at both ends so that no standing waves A still further object of the invention is to proare developed. The connections from the ends vide an improved balanced detector for a radar of the quarter-wave section I 6 through the system oi the frequency modulated type. diodes i8 and It to ground are of sufficiently According to a preferred embodiment of the high impedance to avoid disturbing the impedinvention, signal from the radio transmitter and ance match. The inductance coils 2i and 22 are signal received from the reflecting object are high impedance choke coils that provide directapplied to opposite ends of a line a quarter wave current return paths for the diodes l8 and I9, length long. The two signals are each shifted respectively. The diodes l8 and iii are pro- 90 degrees in phase by the time they travel the vided with cathode resistors 23 and 24, respeclength of the quarter-wave line so there is a 180 25 tively, from which the beat frequency output sigdegree phase difference between the transmitternal is taken. Y receiver signal relation at opposite ends of the The received signal at the end of the section line. Rectifiers at each end of the line provide it feeding the diode it has been shifted 90 debeat frequency outputs and the beat frequency grees with respect to the received signal at the outputs of the two rectifiers are 180 degrees out antenna end of the section it. Likewise, the of phase. The beat frequency outputs are then transmitted signal at the antenna end of secsupplied to a push-pull circuit and only signals tion it has been shifted 90 degrees with respect having the 180 degree relation appear in the cirto the transmitted signal at the other end of cult output. section it.- Therefore, at the opposite ends of The invention will be better understood from the section it there is a 180 degree phase difthe following description taken in connection ference between the transmitter-receiver signal with the accompanying drawing in which relation. Consequently, the beat frequency out- Figure 1 is a block and circuit diagram showputs of the diodes i8 and is resultin from the ing an embodiment of the invention where a sinmixing of the transmitted and received signals gle antenna is employed for both transmitting are 180 degrees out of phase.

and receiving, and The outputs of rectifiers l8 and I9 are sup- Figure 2 is a block and circuit diagram showplied to opposite ends of the primary 26 of a ing the invention as applied to a radar system push-pull transformer 21 so that signal is supwhere separate antennas are employed for transplied to an audio amplifier 28 if the said outputs mitting and receiving. I are 180 degrees out of phase. Thus only the In the several figures similar parts are indidesired beat frequency signal is supplied to the cated by similar reference characters. amplifier 28 and any amplitude modulation that In Fig. 1 the invention is shown applied to amay be on the strong transmitted signal is not frequency modulated radar system comprising supplied to the amplifier 28. The output of the a radio transmitter Ill that is cyclically frequency amplifier 28 may be supplied through an amplimodulated by a modulating wave that may be tude limiter 29 to a frequency counter 3| in the triangular in wave shape, for example. The triusual way.

angular wave is applied from a triangular wave Fig. 2 shows'the invention applied to a fregenerator to a frequency modulator l2 that quency modulated radar system having separate frequency modulates the transmitter l0. antennas, 32 and 33 for transmission and recep- 1 An antenna l3, which may be a dipole in a tion, respectively. In this case the signal apparabolic reflector, is provided for use both as plied from the transmitter ill to the line section I6 is a weak signal that has only sufficient amplitude for beating with the received signal to produce the desired beat frequency output. The beating signal from the transmitter "I may be suitably reduced in amplitudeby an attenuator 34 or, instead, the line or cable section It may be coupled to the transmitter with very loose coupling.

Since the beating signal from the transmitter III is a comparatively weak signal, any undesired amplitude modulation thereon will result in only a small amplitude signal after demodulation as compared with a corresponding signal resulting from demodulation oi the full strength transmitted signal. For this reason the problem of eliminating amplitude modulation signal is not as serious where two antennas are used as where a single antenna is used. Nevertheless, it has been the preferred practice to employ a balanced mixer in the two-antenna systems for the purpose of balancing out any amplitude modulation component. As shown in Fig. 2, the circuit of the present invention may be employed as such a balanced. mixer. Its operation is the same as previously described.

I claim as my invention:

' 1. In combination, a radio transmitter, means for radiating signal from said transmitter toward a-reflecting object, means for receiving said signal after reflection from said object, a transmission line section having a length equal to onequarter the wave length at which said transmitter operates, means for applying signal direct from the transmitter to one end of said quarterwave line, means for applying said received signal to the other end of said quarter-wave line, a pair of rectifiers, means for applying signal from one end of said quarter-wave line to on of said rectifiers, means for applying signal from the other end of said quarter-wave line to the other of said rectifiers whereby the beat frequency outputs of said rectiflers ar 180 degrees out of phase, an output circuit, and means for supplying said rectifier outputs to said output circuit in phase opposition so as to pass only signals that are substantially 180 degrees out of phase at said rectifier output circuits.

2. The invention according to claim 1 wherein means is provided for cyclically frequency modulating said radio transmitter, said wav length at which the transmitter operates being that of the carrier wave or mean Irequency signal.

3. In a radio system utilizing radio waves reflected from a distant object, a radio transmitter, 4

an antenna, a transmission line having a length equal to one-quarter the wave length at which the transmitter operates, means for applying reflected signal from said antenna to one end of said line, means for applying signal direct from said transmitter to th other nd of said line, a rectifier connected to one end of said line whereby it supplies a beat frequency output, a rectifier connected to the other end of said line whereby it supplies a beat frequency output that is 180 degrees out of phase with said first beat frequency output, an output circuit, and means for supplying said two beat frequency outputs in phase opposition to said output circuit whereby-only signals having a 180 degree phase relation are passed by said output circuit.

*4. In combination, a. single antenna for transmission and reception, 3, radio transmitter, means including said' antenna. for receiving signal after reflection from a reflecting object, a transmission line section having a length equal to one-quarter 4 4 q the wave length at which said transmitter operates, means for supplying the transmitter signal to said antenna through said quarter-wave line wherebysaid transmitter signal is applied to one end of said'line and said received signal is applied to the other end 01' said line, a pair of rectiflers, means for applying signal from one end of said quarter-wave line to one of said rectifier-s, means for applying signal from the other end of said quarter-wave line to the other of said rectifiers whereby the beat frequency outputs 01' said rectifiers are degrees out of phase, an output circuit, and means for supplying said rectifier outputs to said output circuit in phase opposition so as to pass only signals that are substantially 180 degrees out of phase at said rectifier output circuits.

5. In a frequency modulated radar system, a single antenna for transmission and reception, a

radio transmitter operating at a mean carrier Irequency, means for cyclically frequency modulating said transmitter, means, including said antenna for receiving signal after reflection from a reflecting object, a transmission line section. having a length equal to one-quarter the wave length at which said transmitter operates, means for supplying the transmitter signal to said antenna through said quarter-wave line whereby said transmitter'signal is applied to one end of said line and said received signal is applied to the other end of said line, a pair or rectiflers, means for applying signal from one end of said quarter-wave line to one oi. said rectlfiers, means for applying signal from the other end of said quarter-wave line to the other of said rectifiers whereby the beat frequency outputs of said rectifiers are 180 degrees out of phase, an output circuit, and means for supplying said rectifier out-, puts to said output circuit in phase opposition so as to pass only signals that are substantially 180 (116152865 out of phase at said rectifier output circ 6. In combination, a pair of antennas for transmission and reception, respectively, a radio transmitter connected to the transmitting antenna, means including the receiving antenna for receiving signal after reflection from a reflecting object, a transmission line section having a length equal to one-quarter the wave length at which said transmitter operates, means for supplying a' small amplitude signal direct from the transmitter to one end of said quarter-wave line, means connecting the receiving antenna to the other end of said line, a pair of rectiflers, means for applying signal from one end of said quarterwave line to one of said rectifiers, means for ap-' plying signal from the other end of said quarterwave line to the other of said rectifiers wher bythe beat frequency outputs of said rectifiers are 180 degrees out of phase, an output circuit, and

, means for supplying said rectifier outputs to said output circuit in phase opposition so as to pass only signals that are substantially 180 degrees out of phase at said rectifier output circuits.

JAMES R. MOORE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,402,421 Lindenblad Jun 18, 1946 2,419,046 WOlfl Apr. 15, 1947 

